This invention relates to a pressure control unit for vehicular hydraulic brake systems with a braking force distributor featuring a piston arrangement prestressed by a control force into its rest position, acted upon by an outlet pressure counteracting the control force, and being movable as a result of which an outlet chamber is enlarged, the braking force distributor further featuring a normally open valve by means of which a connection is locked between an inlet and the outlet chamber as soon as the piston arrangement has moved out of its rest position against the control force.
Such a braking force distributor has come to knowledge from the German Pat. (Dt-PS) No. 1,195,185. The braking force distributor is used for balancing the braking pressures between the front and rear axles of a vehicle to which end it is interconnected in the pressure medium line between the braking pressure source and the rear axle brakes. Its fundamental mode of operation consists in that it allows full action of the pressure generated by the braking pressure source on the rear axle brakes up to a determined pressure level. When this pressure level (change point) has been reached the piston arrangement will move out of its rest position, causing the valve to close and ensuring that - as the pressure is further increasing - no further pressure (braking force limiter) or only an outlet pressure will reach the rear axle which outlet pressure is limited as compared with the inlet pressure (braking force reducer). If a limited pressure increase is to be achieved it is necessary for the piston arrangement to have another surface which can be acted upon by the inlet pressure in the direction of the control force and which is smaller than the surface acted upon by the outlet pressure in the opposite direction. The relationship of these two surfaces will determine the further pressure increase in the outlet chamber. If there is no surface acted upon by the inlet pressure there will be no further pressure increase beyond the change point, i.e. in this case the device is a braking force limiter.
Such a balancing of the braking pressures is necessary to ensure that, with various friction values, all the wheels of a vehicle will approach the lock-up limit as evenly as possible. Considering e.g. a brake system which is designed for a medium-degree braking operation without any such braking pressure balancing, a good friction value and the good braking action hence being possible will lead to a strong dynamic axle load shift as a result of which the rear wheels would already be locked before the highest braking force possible would become effective at the front wheels, while in the event of a poor friction value and of the weak braking action hence being possible conditions are completely reversed due to the small axle load shift which may then be neglected. However, even the braking pressure balancing by means of a braking force distributor cannot completely prevent the wheels from locking. All that is thereby only achieved is that the wheels of one axle are prevented from premature locking.
In order to prevent a wheel lock-up on principle, antiskid control systems have been developed which among others feature a pressure control device. Such an antiskid control system has come to knowledge from the German Printed Patent Application (Dt-AS) No. 1,530,471. The pressure control device disclosed there has a plunger which is held in its rest position by a strong spring and which is displaceable by means of a controlled auxiliary pressure against the force of the spring, thereby a control chamber being enlarged and at the same time a valve being closed which is mechanically controlled by the plunger. The valve prevents any further braking pressure supply from the pressure medium source to the control chamber while in the chamber and in the wheel brake connected therewith the braking pressure is reduced by the increase in volume of the control chamber. To this end, the auxiliary pressure is controlled such as to bring about a braking pressure which will just prevent the wheels from locking.
Up to now it was assumed that in vehicles equipped with an antiskid control system the braking pressure balancing is of minor importance since the antiskid control system provides for a better prevention of a wheel lock-up than does a braking force distributor. It was thus assumed that the braking force distributor is dispensable in such vehicles. This, however, leads to a response of the antiskid control system which is connected with the wheels of the dynamically load-relieved axle even if the optimum braking pressure has not yet been reached at the wheels of the other axle. Upon any stronger braking operation, the wheels of the dynamically load-relieved axle are thus strained up to the lock-up limit and overbraking will only be prevented by the continuous operation of the antiskid control system. This in itself is disadvantageous enough. However, if in addition one thinks of the fact that with antiskid control systems the possibility of failure has to be considered and that in such a case the brake system is to work as if no antiskid control system were provided at all, then this is a state which is intolerable. In such a case, upon the failure of the antiskid control system the wheels of the dynamically load-relieved axle will very soon be overbraked as a result of which the vehicle will normally skid since it is the rear wheels that are affected.
Thus one arrives at the conclusion that despite the installation of an antiskid control system a good braking pressure balancing is needed. The most simple way to provide such an optimum brake system is to provide both devices, i.e. an antiskid control system and a braking force distributor. In consequence, however, the entire brake system will become very voluminous and expensive.